The field of the present invention relates to optical elements. In particular, optical elements are disclosed herein that enable free-space optical propagation between an optical waveguide and another optical waveguide, component, or device.
Planar optical waveguides are suitable for implementing a variety of optical devices for use in telecommunications and other fields. For the purposes of the present disclosure and appended claims, the term “planar optical waveguide” is intended to generally encompass waveguide structures deposited or otherwise formed on a substantially planar substrate. Optical paths defined by or among such planar waveguides can be arranged in two or three dimensions. In addition to the planar waveguides, the planar waveguide substrate often also includes (by fabrication and/or placement thereon): alignment or support structures for placement of optical components or devices on the substrate; V-grooves or other alignment or support structures for positioning of optical fibers and/or fiber-optic tapers on the substrate; compensators, gratings, filters, or other optical components, elements, or devices; electrical contacts or traces for enabling electronic access to active devices on the substrate; or other suitable components. Reflective or transmissive optical elements including, but not limited to, mirrors, beamsplitters, beam combiners, filters, lenses, and so forth are disclosed herein for use with one or more planar optical waveguides that enable free-space optical propagation between an optical waveguide and another optical waveguide, component, or device.
Many of the optical waveguides (including both optical fibers and planar waveguides) described herein have dimensions and design parameters so as to support only one or a few lowest-order optical modes. At visible and near-IR wavelengths (e.g., those typically employed for optically-based telecommunications), the resulting optical modes are typically a few μm up to about 10 or 15 μm in transverse extent. Depending on the nature of the waveguide, the guided optical mode(s) may be nearly cylindrically symmetric, or may differ substantially in transverse extent along substantially orthogonal transverse dimensions. Modes of these wavelengths and sizes typically exhibit diffractive behavior upon emerging from the end face of the supporting waveguide and propagating as an optical beam, typically (but not always) becoming substantially divergent sufficiently far from the end face of the supporting waveguide (NA often greater than about 0.1). Accordingly, one or more of the following adaptations may be required to achieve a degree of optical power transfer above an operationally acceptable level between an end-coupled waveguide and another optical component or device: maintain the unguided optical pathlength between the waveguide and the other waveguide, component, or device as small as practicable for a particular optical assembly; adapt the end portion of the waveguide or the other waveguide, component, or device for mitigating the diffractive behavior of the optical beam beyond the waveguide; or insert one or more additional optical elements between the waveguide and the other waveguide, component, or device for refocusing, re-imaging, or otherwise manipulating the beam spatial properties for enhancing end-coupling between the waveguide and the other component or device.
It is often the case in a waveguide-based optical system or in a waveguide-based multi-component optical device that optical functionality is to be provided that cannot be readily implemented within a waveguide, and must therefore be provided by a reflective or transmissive optical element interposed in the optical path wherein an optical signal propagates as an optical beam (reflected from a reflective optical element or transmitted through a transmissive optical element). In order to implement optical functionality in this way while maintaining overall transmission through the optical system at or above an operationally acceptable level, it is typically necessary to adapt the optical system or multi-component optical device as described in the preceding paragraphs.
For purposes of the present disclosure or appended claims, the term “optical beam” shall denote so-called free-space propagation of an optical signal, determined by the diffractive behavior of electro-magnetic waves and substantially unconfined by any sort of refractive index variation, gradient, or structure that would result in waveguide-like behavior. Such free-space optical propagation can occur through vacuum, through air or another gaseous medium, through a liquid medium, or through a solid medium. In contrast, propagation of an optical signal that is confined or guided in at least one transverse dimension by a refractive index variation, gradient, or structure acting as a waveguide shall be referred to herein as an “optical mode” or a “guided mode.”
The subject matter of the present application may be related to subject matter disclosed or claimed in: U.S. Pat. No. 7,031,575; U.S. Pat. No. 7,142,772; and U.S. Pat. No. 7,366,379. Each of said patents is hereby incorporated by reference as if fully set forth herein.